Our understanding of tumor angiogenesis can be traced back for over two hundred years. However, it was not until recently (2004;FDA approved) that the first angiogenic inhibitor, bevacizumab (AvastinTM), was shown to increase overall survival (OS) in patients with colorectal cancer when combined with chemotherapy. Since this initial breakthrough, clinical studies targeting other cancers have been unable to reproduce these results. In patients with metastatic breast cancer, combining bevacizumab with chemotherapy did not improve OS and in many cases favored the placebo arm (AVADO and RIBBON studies) in spite of observing an increase in progression free survival (PFS). Clearly angiogenic inhibitors have demonstrated great promise, but their role remains to be fully elucidated. To address this lack of efficacy, we propose that therapeutic regimens of angiogenic inhibitors initially delay tumor growth by targeting its vasculature, but they also increase tumor and tissue hypoxia, which in turn, expand cancer stem cell (CSC) populations and increase therapeutic resistance and potentiate metastasis. If substantiated, our hypothesis would explain the temporary gains (PFS) but poor outcome. Given that distant metastatic failure in breast cancer patients leads to poor clinical outcome, particular emphasis will be placed on identifying breast cancer stem cells within regions of acute hypoxia associated with metastasis and targeting these regions by devising new regimens of angiogenic inhibitors. Realizing that ex vivo or in vitro cancer studies cannot reliably model the complex spatial-temporal variations in tumor oxygen levels, a novel Multivariate in vivo Hemodynamic imaging Model of Oxygen, dubbed MiHMO2, is proposed. MiHMO2 quantifies local variations in tissue hemodynamics by measuring the hemoglobin status (CtHb, SaO2) using Photoacoustic Computed Tomographic Spectroscopy and physiological state (perfusion, permeability, vascular and cellular volumes) using Dynamic Contrast-Enhanced CT within the tumor, and then fusing this information based on a mathematical model to obtain tissue local values of pO2, hypoxic fraction (HF), and acute and chronic types of hypoxia. With this unique capability, a new therapeutic regimen will be investigated: (Specific Aim 1) investigate whether pO2 levels influence the breast cancer stem cell niche and it's subpopulations in vivo, and (Specific Aim 2) investigate how alteration of pO2 levels and the tumor hypoxic fraction by anti-angiogenic influences the size of the BCSC niche and it's subpopulations. We will use MiHMO2 measure local pO2 in breast tumors (MCF-7, MCF-7VEGF, MDA-MB-231, BT474), to categorize regions of acute or chronic hypoxia, and to identify the subset of breast cancer stem cell biomarkers associated with metastasis. With this knowledge, a dose regimen of angiogenic inhibitors will be devised to reduce the hypoxic-niche associated with metastatic BCSCs, and determine whether improved pO2 levels alter the size BCSC niche and/or the composition of BCSC subpopulations. These goals are innovative and directly address an important clinical need, which can be extended to nearly all tumor types. PUBLIC HEALTH RELEVANCE: Current therapeutic regimens of angiogenic inhibitors in breast cancer patients have not increased overall survival, but instead are believed to increase tumor and tissue hypoxia, which, in turn, expand breast cancer stem cell (BCSC) populations that resist therapy and potentiate metastasis. A new imaging modality is proposed to non invasively measure tumor hypoxia and to distinguish between acute and chronic hypoxia, where the former is hypothesized to harbor a metastatic subpopulation of BCSCs. New regimens of angiogenic inhibitors will be investigated to reduce this subpopulation and distant metastasis, and thus increase overall survival.